The present invention relates to a method for processing used oil comprising approximately 70 percent by weight spindle and neutral oils, 5 percent by weight lower and higher boiling point oils, 10 percent by weight additives and oxidation products, 10 percent by weight water as well as 5 percent by weight difficultly volatile or nonvolatile hydrocarbons and varying contents of solids. The solids contained in the used oil, are, for example, sand, dust and metal particles. The secondary raffinate obtained from processing of used oil is used particularly as lubricating oil.
In a known method for processing used oil, an aqueous phase and the solids are initially separated from the used oil by sedimentation in storage vessels. Then, solvents, emulsified water and light oils are distilled from the oil-containing phase. The distillation residue, which is called dry oil, is then mixed, for purposes of raffination, with 12 percent by weight concentrated sulfuric acid. The resulting material is the so-called acid resin which is separated in the form of a sludge from the raffinate, which is called the acid oil. After the addition of 5 percent by weight bleaching earth to the acid oil, a vacuum distillation is performed providing heavy oil and spindle oil. A basic oil is produced from part of the spindle oil and the heavy oil, to which suitable additives are added in order to permit processing into machine and engine oil. After neutralization, the raffination residue (that is, the acid resin and the residue from the distillation of the acid oil) is burnt or stored in a suitable depository.
According to another prior art method, dry oil is mixed with lime, decomposing a great many components foreign to oil but also desirable additives. The decomposition products are bound, polymerized or coagulated. By subsequent vacuum distillation, raffinates of a high degree of purity are obtained which must be further purified by subsequent raffination with sulfuric acid, bleaching earth or extraction with a solvent.
In still another prior art method, extraction of dry oil with liquid propane has also been proposed. The liquid propane charged with oil is extracted at the head of an extraction column while the insoluble residue flows from the bottom of the column. After recovery of the propane still contained therein, the residue is burnt together with heating oil. The propane is separated from the oil-containing extract by means of a two-stage evaporation process and, after liquefaction, is returned to the extraction column. The extracted oil, free of propane, is subjected to sulfuric acid raffination.
The drawback of sulfuric acid raffination is, in particular, that a large quantity of sulfuric acid (12 percent by weight of the dry oil) is required and that considerable quantities of waste sulfuric acid, acid resin or acid ether (about 36 percent by weight of the dry oil), as well as sulfur dioxide, are obtained. The disposal of the acid resin proves to be particularly difficult since it must be neutralized before it can be stored in a depository and since its combustion produces large quantities of sulfur dioxide. The use of lime or propane for processing used oil also results in the formation of waste products which are difficult to dispose of.
The critical temperature of a gas is well known to physical chemists as the temperature above which the gas cannot be liquified regardless of the pressure to which it is subjected. The pressure necessary to liquify the gas at its critical temperature is the critical pressure.
It is known that gases in the supercritical state have the capacity to absorb certain compounds depending on their precise thermodynamic state. A change in thermodynamic state can result in the release of the absorbed substances.
German Auslegeschrift No. 1,493,190 discloses a method for processing mixtures which are liquid and/or solid under process conditions, and which contain organic compounds and/or organic groups. The mixture of substances is treated with a gas under supercritical conditions of temperature and pressure, at a temperature up to 100.degree. C. beyond the critical temperature of the gas. After separation of the supercritical gas phase, the compounds contained therein are recovered by relaxation and/or increased temperature. Spent machine oil was treated with ethylene gas according to this method, and it was reported that 90 percent of the oil could be separated. However, no information is given about the composition of the extracted product, and it would not be expected that the additives contained in the used oil would also be extracted, and that both the extracted oils and the extracted additives could be reused.